Let
N
be the effective power radiated from an
isotropic antenna
and
p
be the power density at a distance
d
from this source
[1]
-
Power density is also defined in terms of electrical field strength;
Let
E
be the electrical field and
Z
be the impedance of the free space
-
The following relation is obtained by equating the two,
-
or by rearranging the terms
-
Impedance of free space
is roughly
Since a
half wave dipole
is used, its
gain
over an isotropic antenna (
) should also be taken into consideration,
-
In this equation
SI units
are used.
Expressing the same equation in:
- kW
instead of
W
in
power
,
- km
instead of
m
in
distance
and
- mV/m
instead of
V/m
in
electric field
is equivalent to multiplying the expression on the right by
.
[2]
In this case,
-
- ^
Reference data for radio Engineers
, Howard W.Sams co, Indianapolis, 1956, 27-7
- ^
K.H.Kaltbeitzer:
Site selection
, EBU Techhnical Monograph 3104, Bruxelles, 1965, p 30